Powders for ceramic permanent magnetics have been fabricated by a variety of methods in the prior art. Traditionally, these iron oxide powders were formed by sintering an admixture of iron oxide with other oxides or compounds which yield the desired composition. Although the particles finally produced were magnetic, they were much too large as a result of the extensive heat treating for service in fine particle magnetic applications such as magnetic recording media.
In order to overcome these limitations, several methods of producing permanent magnetic materials, such as barium or strontium ferric oxides, in high temperature environments rather than by admixing and sintering have been developed by the prior art. British Patent No. 888,688 teaches a method of making fine barium ferrites by injecting atomized iron and barium salt solutions into an oxidizing flame at temperatures between 1000.degree. C. and 1400.degree. C. The powder produced by this process does not, however, possess the M-phase crystalline form but instead consists of mixed or indeterminant phase particles exhibiting only soft magnetic properties. A subsequent heat treating step at a temperature above 600.degree. C. is therefore required to convert the mixed phase particles to the desired M-phase. Because this heat treatment must be applied to powders in bulk, extreme caution must be practiced to ensure that localized sintering of the single powder particles does not take place, otherwise the powder's size, shape and aggregation will be adversely affected.
A method of producing metal oxide compounds by passing feed solution droplets in countercurrent flow through hot gases was disclosed in U.S. Pat. No. 3,378,335. Decomposition of the metal salt droplets occurred at temperatures between 600.degree. C. and 800.degree. C. which are much too low to promote formation of particles having the M-phase structure. The powder produced by this method consisted of mixtures of fine particles of different crystal phases. Consequently, even after subsequent heat treatment, the particles exhibited minimal coercive field strength and saturation magnetization.
In a publication entitled "Ultrafine Metal Oxides by Decomposition of Salts in a Flame", Proceeding of the Electrochemical Society Symposium--Ultrafine Particles, May 3, 1961, pp. 181-195, the authors disclose that a large number of mixed metal oxides, including barium ferrite, may be formed by injecting metal salts into a flame environment. No processing steps or parameters for forming the barium ferrite are delineated, nor is the crystal structure of the ferrite particles recited. Hence, the reference fails to teach or suggest any method steps or conditions by which the M-phase crystal structure may be formed.
Barium or strontium ferrite particles possessing the M-phase crystal structure were produced by the high temperature decomposition of a metal salt solution in U.S. Pat. No. 4,336,242. In the practice of this method, barium or strontium ferrous salt solutions are fed into a flame and then substantially converted to an M-phase barium or strontium ferrite. It is essential, according to the teaching of this patent, that each salt particle be formed directly from a single solution droplet. No local demixing of the individual components can occur; hence, the homogeneous distribution of the substances dissolved in solution is retained during the entire reaction. In order to maintain the desired homogeny, it is imperative that the reaction be carried out at an extremely rapid rate. Consequently, the process requires the use of low vapor pressure ferrous chloride, mixed chloride nitrate salts and powder temperatures limited to 1,173.degree. K. Moreover, since the size of the M-phase particle is dependent upon the original droplet size, the process is incapable of producing fine, unaggregated M-phase particles. Under these conditions, the particles produced are large, on the order of 1 to 10 microns, and are composed of various segments of different crystalline orientations which require a subsequent grinding process to separate the individual primary grains. Hence, the process disclosed in U.S. Pat. No. 4,336,242 fails to produce fine, single ferrite particles having the M-phase crystal structure.